In aviation lighting there are stringent requirements on the amount of light that must be provided to meet minimum safety standards in some installations. In others, there are customer requirements that dictate how much light must be available. It is therefore desirous to know if any and how many light emitting diodes (LEDs) being used in a particular light are functioning properly.
In high power LED general lighting implementations it is very desirous to place the LEDs in a single string so as to reduce the number electronic devices required to provide the proper current to the LEDs. The simplest way to verify that a single LED is functioning properly is to monitor the voltage across the LED when the proper drive current is applied. Complications with this method include several factors. Two well-known factors are the forward voltage of an LED is very sensitive to the junction temperature of the LED die and to the applied drive current. Thus over the temperature extremes that an LED will be exposed to on the outside of an aircraft, a wide range of voltage can be seen. Likewise, the forward voltage will see a wide range based on small changes of applied current. Also, each LED has a potential inherent difference in forward voltage from each other at the same current level. This limits the number of LEDs in a string that can be monitored effectively without additional complexity.
One method of monitoring is to use a relative measurement where points in the LED string are tapped into for voltage measurement and are compared relative to the LED directly adjacent to determine if the LEDs are within the expected voltage range for the applied current. The temperature variation and the normal LED forward voltage variation conspire to limit the number of LEDs that can be sampled to 1 or 2. The disadvantage of this method is the large amount of electrical components needed to monitor a string of more than 4 LEDs.